Hetero junction field effect transistor and method of fabricating the same

ABSTRACT

There is provided a hetero junction field effect transistor including: a first layer of a nitride based, group III-V compound semiconductor; a second layer of a nitride based, group III-V compound semiconductor containing a rare earth element, overlying the first layer; a pair of third layers of a nitride based, group III-V compound semiconductor, overlying the second layer, the third layers being spaced from each other; a gate electrode disposed between the third layers at least a region of the second layer; and a source electrode overlying one of the third layers and a drain electrode overlying an other of the third layers. A method of fabricating the hetero junction field effect transistor is also provided.

This application is a divisional application of U.S. patent applicationSer. No. 11/806,783 filed Jun. 4, 2007, claiming priority to JapanesePatent Application No. 2006-176347 filed with the Japan Patent Office onJun. 27, 2006, the entire contents of all of which are herebyincorporated by reference.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to hetero junction field effecttransistors and methods of fabricating the same, and particularly tohetero junction field effect transistors allowing a recessed gate to beformed with good repeatability and methods of fabricating the same.

2. Description of the Background Art

Conventionally a normally off hetero junction field effect transistorutilizing a nitride based, group III-V compound semiconductorrepresented by a formula of Al_(x)Ga_(y)In_(z)N has been implemented forexample by etching a barrier layer of nitride based, group III-Vcompound semiconductor underlying a gate electrode in plasma to reducethe layer in thickness to form a recessed gate, wherein 0≦x≦1, 0≦y≦1,0≦z≦1, and x+y+z≠0 (see “Normally Off AlGaN/GaN HEMT with Recessed Gatefor High Power Applications”, Ken Nakata, Takeshi Kawasaki, and SeijiYaegassi, IEICE Technical Report, Vol. 105, No. 325, pp. 51-56, forexample).

SUMMARY OF THE INVENTION

However, conventionally etching in plasma, as described above, isdisadvantageous in that etching the barrier layer in plasma is done ascontrolled in time.

More specifically, even if the state of the plasma varies with thecurrent condition of equipment, wafers are etched in the plasma only fora fixed period of time. As a result, the wafers thus processed in theplasma are etched differently in depth and provide barrier layers havingtheir respective thin portions different in thickness.

Furthermore, even a single wafer thus etched also has a depth varyingwith the state of the plasma, and thus provides a barrier layer having athin portion varying in thickness.

Thus while forming a recessed gate requires that a barrier layer have athin portion with a thickness controlled with a precision of a few nmwith good repeatability, the conventional method that utilizes etchingin plasma provides a single wafer with a barrier layer having a thinportion varying in thickness, and different wafers with barrier layershaving thin portions, respectively, different in thickness. Thisdisadvantageously provides recessed gates with variation and hencehetero junction field effect transistors with variation incharacteristics.

The present invention contemplates a hetero junction field effecttransistor allowing a recessed gate to be formed with good repeatabilityand a method of fabricating such hetero junction field effecttransistor.

The present invention is a hetero junction field effect transistorincluding: a first layer of a nitride based, group III-V compoundsemiconductor; a second layer of a nitride based, group III-V compoundsemiconductor containing a rare earth element, overlying the firstlayer; a pair of third layers of a nitride based, group III-V compoundsemiconductor, overlying the second layer, the third layers being spacedfrom each other; a gate electrode disposed between the third layers atleast a region of the second layer; and a source electrode overlying oneof the third layers and a drain electrode overlying an other of thethird layers.

In the present hetero junction field effect transistor preferably therare earth element is at least one selected from the group consisting ofCerium (Ce), praseodymium (Pr), neodymium (Nd), promethium (Pm),samarium (Sm), europium (Eu), gadolinium (Gd), terbium (Tb), dysprosium(Dy), holmium (Ho), erbium (Er), thulium (Tm), ytterbium (Yb) andlutetium (Lu).

In the present hetero junction field effect transistor preferably thethird layer is formed of an n type, nitride based, group III-V compoundsemiconductor.

In the present hetero junction field effect transistor preferably thesecond layer has a thickness of at most 5 nm.

Furthermore the present invention is a hetero junction field effecttransistor including: a first layer of a nitride based, group III-Vcompound semiconductor; a second layer of a nitride of a rare earthelement, overlying the first layer; a pair of third layers of a nitridebased, group III-V compound semiconductor, overlying the second layer,the third layers being spaced from each other; a gate electrode disposedbetween the third layers at least a region of the second layer; and asource electrode overlying one of the third layers and a drain electrodeoverlying an other of the third layers.

In the present hetero junction field effect transistor preferably therare earth element is at least one selected from the group consisting ofCe, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb and Lu.

In the present hetero junction field effect transistor preferably thethird layer is formed of an n type, nitride based, group III-V compoundsemiconductor.

In the present hetero junction field effect transistor preferably thesecond layer has a thickness of at most 5 nm.

Furthermore the present invention is a method of fabricating a heterojunction field effect transistor, including the steps of: depositing ona first layer of a nitride based, group III-V compound semiconductor asecond layer of one of a nitride based, group III-V compoundsemiconductor containing a rare earth element or a nitride of a rareearth element; depositing on the second layer a third layer of a nitridebased, group III-V compound semiconductor; providing a source electrodeand a drain electrode on the third layer; etching away a portion of thethird layer in a plasma with a gas containing one of chlorine andfluorine to expose a portion of a surface of the second layer; andproviding a gate electrode on at least a region of the surface of thesecond layer exposed.

The present invention can thus provide a hetero junction field effecttransistor allowing a recessed gate to be formed with good repeatabilityand a method of fabricating such hetero junction field effecttransistor.

The foregoing and other objects, features, aspects and advantages of thepresent invention will become more apparent from the following detaileddescription of the present invention when taken in conjunction with theaccompanying drawings.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a schematic cross section of one preferred example of thepresent hetero junction field effect transistor.

FIGS. 2-4 are schematic cross sections for illustrating one example ofsteps in fabricating the hetero junction field effect transistor of FIG.1.

FIG. 5 is a schematic cross section of another preferred example of thepresent hetero junction field effect transistor.

FIGS. 6-8 are schematic cross sections for illustrating one example ofsteps in fabricating the hetero junction field effect transistor of FIG.5.

DESCRIPTION OF THE PREFERRED EMBODIMENTS

Hereinafter embodiments of the present invention will be described. Inthe figures identical reference characters denote identical orcorresponding portions.

First Embodiment

FIG. 1 is a schematic cross section of one preferred example of thepresent hetero junction field effect transistor. The FIG. 1 heterojunction field effect transistor has the structure including a multibuffer layer 2 having an AlN layer and a GaN layer alternately depositedin layers, a GaN layer 3 serving as a channel layer, an AlN layer 4serving as a hetero characteristic improvement layer, anAl_(0.3)Ga_(0.7)N layer 5 serving as a first layer, anAl_(0.35)Ga_(0.6)Gd_(0.05)N layer 6 serving as a second layer, and anAl_(0.3)Ga_(0.7)N layer 7 serving as a pair of third layers deposited onas Si substrate 1 in this order.

The pair of Al_(0.3)Ga_(0.7)N layers 7 are spaced onAl_(0.35)Ga_(0.6)Gd_(0.05)N layer 6. On one Al_(0.3)Ga_(0.7)N layer 7 asource electrode 8 is disposed and on the other Al_(0.3)Ga_(0.7)N layer7 a drain electrode 10 is disposed. Furthermore a gate electrode 9 isdisposed between Al_(0.3)Ga_(0.7)N layers 7 at least a portion of theregion of Al_(0.35)Ga_(0.6)Gd_(0.05)N layer 6.

AlN layer 4 has a band gap larger than GaN layer 3. Accordingly, a twodimensional electron gas layer is provided at an interface of GaN layer3 and AlN layer 4 at a portion closer to GaN layer 3 to serve as achannel region. Source electrode 8 feeds a carrier which passes throughthe two dimensional electron gas layer serving as a channel region andis extracted from drain electrode 10. Whether the carrier is movedthrough the channel region can be controlled by whether voltage isapplied to gate electrode 9.

The FIG. 1 hetero junction field effect transistor can be fabricated forexample as follows: Initially, as shown in the FIG. 2 showing aschematic cross section, on Si substrate 1, multi buffer layer 2, GaNlayer 3 having a thickness for example of 2 μm, AlN layer 4 having athickness for example of 1 nm, Al_(0.3)Ga_(0.7)N layer 5 having athickness for example of 10 nm, Al_(0.35)Ga_(0.6)Gd_(0.05)N layer 6having a thickness for example of 3 nm, and Al_(0.3)Ga_(0.7)N layer 7for example having a thickness of 20 nm and a carrier density of 5×10¹⁸cm⁻³ are grown for example by MOCVD, MBE, or similar vapor phasedeposition method. If Al_(0.35)Ga_(0.6)Gd_(0.05)N layer 6 is grown forexample by MOCVD, a Gd source implemented by Cp3Gd (cyclopenta Gd) canbe used, and if the layer is grown for example by MBE, a Gd sourceimplemented by a simple substance of metal of Gd can be used.

The substrate is not limited to Si substrate 1. It may be a sapphiresubstrate, an SiC substrate or a similar semiconductor substrate. Notethat multi buffer layer 2 has a layered structure that can vary with thetype of the substrate.

Furthermore GaN layer 3 serving as a channel layer preferably has athickness of at least 1 μm. Furthermore the channel layer preferably hasa carrier density reduced as much as possible. For example the layerpreferably has a carrier density of at most 10¹⁵ cm⁻¹.

Furthermore, Al_(0.3)Ga_(0.7)N layer 5 serving as the first layer is notlimited to any particular composition or thickness. It should be noted,however, that the layer varies in thickness with composition to have athickness required to provide a normally off transistor. For example forAl_(0.3)Ga_(0.7)N layer 5 a thickness of at most 10 nm allows atransistor to be a normally off transistor.

Furthermore, the second layer is not limited to any particularcomposition or any particular number of constituent rare earth elements.Furthermore Al_(0.3)Ga_(0.7)N layer 7 serving as the third layer is notlimited to any particular thickness or any particular carrier density.

Photolithography is then employed to provide resist patterned in apredetermined form on a surface of Al_(0.3)Ga_(0.7)N layer 7, and a filmof metal is for example vapor-deposited thereon for the source and drainelectrodes. The resist is then lifted off and thus removed andthereafter the film of metal remaining on the surface ofAl_(0.3)Ga_(0.7)N layer 7 undergoes a heat treatment. Thus, as shown inFIG. 3 showing a schematic cross section, source and drain electrodes 8and 10 are provided on the surface of Al_(0.3)Ga_(0.7)N layer 7, andsource and drain electrodes 8 and 10 each have ohmic contact withAl_(0.3)Ga_(0.7)N layer 7.

Note that source and drain electrodes 8 and 10 can be formed of a filmof metal that is formed of a Ti layer and an Al layer deposited in thisorder, a film of metal that is formed of a Hf layer, an Al layer, a Hflayer, and an Au layer deposited in this order, or the like.

Subsequently Al_(0.3)Ga_(0.7)N layer 7 is partially etched away inplasma to partially expose a surface of Al_(0.35)Ga_(0.6)Gd_(0.05)Nlayer 6 to form a recessed gate, as shown in FIG. 4 showing a schematiccross section. Al_(0.3)Ga_(0.7)N layer 7 can be etched in plasma forexample in chlorine gas by an inductively coupled plasma (ICP) etcher.

On at least a portion of the exposed surface ofAl_(0.35)Ga_(0.6)Gd_(0.05)N layer 6 gate electrode 9 formed for exampleof WN is disposed to have schottky contact withAl_(0.35)Ga_(0.6)Gd_(0.05)N layer 6. A wafer having provided with gateelectrode 9 is divided into chips to provide the hetero junction fieldeffect transistor shown in FIG. 1.

In the present embodiment Al_(0.3)Ga_(0.7)N layer 5,Al_(0.35)Ga_(0.6)Gd_(0.05)N layer 6 and Al_(0.3)Ga_(0.7)N layer 7provide a 3-layer barrier layer. The center, Al_(0.35)Ga_(0.6)Gd_(0.05)Nlayer 6 is thus etched in plasma at a rate significantly smaller thanAl_(0.3)Ga_(0.7)N layer 7. If the layer is etched in plasma at aslightly varying rate or for a slightly increased period of time, theunderlying, Al_(0.3)Ga_(0.7)N layer 5 is not etched in plasma. This canleave Al_(0.3)Ga_(0.7)N layer 5 well controlled to have a thicknessrequired for normally off operation, and thus form a recessed gate withgood repeatability.

The hetero junction field effect transistor thus fabricated can be anormally off hetero junction field effect transistor excellent inin-plane uniformity (i.e., the uniformity of the voltage (0V fornormally off) being applied to the gate electrode of each heterojunction field effect transistor that is obtained from a single waferwhen the transistor is pinched off) and run-to-run uniformity (i.e., theuniformity of the voltage (0V for normally off) being applied to thegate electrode of each of hetero junction field effect transistors thatare obtained from different wafers identical in structure when thetransistor is pinched off).

Note that Al_(0.3)Ga_(0.7)N layer 7 can be etched in plasma for a periodof time that can be determined as follows: A period of time allowingAl_(0.3)Ga_(0.7)N layer 7 to be etched in plasma entirely in depth ispreviously calculated and for example the calculated period of time isthen increased by a few tens percents, and for such a longer period oftime than the previously calculated period of time the layer may beetched in plasma. For example, if a period of 30 seconds is required toetch Al_(0.3)Ga_(0.7)N layer 7 having a thickness of 15 nm in plasma,the layer is etched in plasma for 36 seconds. AsAl_(0.35)Ga_(0.6)Gd_(0.05)N layer 6 acts as an etching stopper layer,the underlying, Al_(0.3)Ga_(0.7)N layer 5 can remain to have apredetermined thickness uniformly.

Furthermore while in the above description the second layer isimplemented by Al_(0.35)Ga_(0.6)Gd_(0.05)N layer 6, the second layer ispreferably implemented by a layer of material containing a nitridebased, group III-V compound semiconductor represented by a formula ofAl_(x)Ga_(y)In_(z)N containing at least one type of rare earth elementselected from the group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb,Dy, Ho, Er, Tm, Yb and Lu, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≠0. Anitride of these rare earth elements provides semiconductor. Accordinglythe second layer formed of a nitride based, group III-V compoundsemiconductor represented by the formula of Al_(x)Ga_(y)In_(z)Ncontaining at least one type of the rare earth elements can also besemiconductor, wherein 0≦x≦1, 0≦y≦1, 0≦z≦1, and x+y+z≠0. Thus the secondlayer thus configured also does not impair the transistor incharacteristic and also has a tendency to serve as an effective etchingstopper layer.

Furthermore in the present invention preferably the second layer formedof the nitride based, group III-V compound semiconductor containing therare earth element(s) has a thickness of at most 5 nm. If the secondlayer has a thickness exceeding 5 nm it would sufficiently serves as anetching stopper layer. However, this provides a tendency that a layerunderlying a gate electrode has a thickness larger than required toprovide a normally off transistor, and such transistor may not beprovided.

Furthermore in the present invention preferably the third layer isformed of an n-type, nitride based, group III-V compound semiconductor.If the third layer is formed of the n-type, nitride based, group III-Vcompound semiconductor, there is a tendency that the ohmic contact withthe source and drain electrodes disposed on the third layer can bereduced in resistance. This can contribute to reduced on-stateresistance. Furthermore if the second layer has a band gap smaller thanthe third layer the third layer is preferably formed of n type, nitridebased, group III-V compound semiconductor to supply sufficient electronsto supplement those trapped in the second layer.

Second Embodiment

FIG. 5 is a schematic cross section of one preferred example of thepresent hetero junction field effect transistor. The FIG. 5 heterojunction field effect transistor has the structure, multi buffer layer 2having an AlN layer and a GaN layer alternately deposited in layers, anAl_(0.05)Ga_(0.95)N layer 13 serving as a lower barrier layer, a GaNlayer 14 serving as a channel layer, an AlN layer 15 serving as a heterocharacteristic improvement layer, an Al_(0.25)Ga_(0.7)In_(0.05)N layer16 serving as a first layer, a DyN layer 17 formed of a nitride of Dy,which is a rare earth element, or dysprosium nitride (DyN), and servingas a second layer, and an Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 servingas a pair of third layers.

The pair of Al_(0.3)Ga_(0.65)In_(0.05)N layers 18 are spaced on DyNlayer 17. On one Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 source electrode 8is disposed and on the other Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 drainelectrode 10 is disposed. Furthermore gate electrode 9 is disposedbetween Al_(0.3)Ga_(0.65)In_(0.05)N layers 18 at least a portion of theregion of DyN layer 17.

Al_(0.05)Ga_(0.95)N layer 13 has a band gap larger than GaN layer 14.Accordingly, a two dimensional electron gas layer is provided at aninterface of Al_(0.05)Ga_(0.95)N layer 13 and GaN layer 14 at a portioncloser to GaN layer 14 to serve as a channel region. Source electrode 8feeds a carrier which passes through the two dimensional electron gaslayer serving as a channel region and is extracted from drain electrode10. Whether the carrier is moved through the channel region can becontrolled by whether voltage is applied to gate electrode 9.

The hetero junction field effect transistor shown in FIG. 5 can befabricated for example as follows: Initially, as shown in the FIG. 6showing a schematic cross section, on Si substrate 1, multi buffer layer2, Al_(0.05)Ga_(0.95)N layer 13 having a thickness for example of 2 μm,GaN layer 14, AlN layer 15 having a thickness for example of 1 nm,Al_(0.25)Ga_(0.7)In_(0.05)N layer 16 having a thickness for example of10 nm, DyN layer 17 having a thickness for example of 2 nm, andAl_(0.3)Ga_(0.65)In_(0.05)N layer 18 for example having a thickness of25 nm and a carrier density of 5×10¹⁸ cm⁻³ are grown for example byMOCVD, MBE, or similar vapor phase deposition method. If DyN layer 17 isgrown for example by MOCVD, a Dy source implemented by Cp3Dy (cyclopentaDy) can be used, and if the layer is grown for example by MBE, a Dysource implemented by a simple substance of metal of Dy can be used.

The substrate is not limited to Si substrate 1. It may be a sapphiresubstrate, an SiC substrate or a similar semiconductor substrate. Notethat multi buffer layer 2 has a layered structure that can vary with thetype of the substrate.

Furthermore Al_(0.05)Ga_(0.95)N layer 13 serving as a lower barrierlayer preferably has a thickness of at least 1 μm. Furthermore thechannel layer preferably has a carrier density reduced as much aspossible. For example the layer preferably has a carrier density of atmost 10¹⁵ cm⁻³.

Furthermore, Al_(0.25)Ga_(0.7)In_(0.05)N layer 16 serving as the firstlayer is not limited to any particular composition or thickness. Itshould be noted, however, that the layer varies in thickness withcomposition to have a thickness required to provide a normally offtransistor. For example for Al_(0.25)Ga_(0.7)In_(0.05)N layer 16 athickness of at most 10 nm allows a transistor to be a normally offtransistor.

Furthermore, the second layer is not limited to any particularcomposition or any particular number of constituent rare earth elements.The second layer may by other than the DyN layer. It may for example bea Gd_(0.8)Dy_(0.2)N layer, a Ce_(0.2)Pr_(0.6)Nd_(0.2)N layer, or thelike.

Furthermore Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 serving as the thirdlayer is not limited to any particular thickness or any particularcarrier density.

Photolithography is then employed to provide resist patterned in apredetermined form on a surface of Al_(0.3)Ga_(0.65)In_(0.05)N layer 18,and a film of metal is for example vapor-deposited thereon for thesource and drain electrodes. The resist is then lifted off and thusremoved and thereafter the film of metal remaining on the surface ofAl_(0.3)Ga_(0.65)In_(0.05)N layer 18 undergoes a heat treatment. Thus,as shown in FIG. 7 showing a schematic cross section, source and drainelectrodes 8 and 10 are provided on the surface ofAl_(0.3)Ga_(0.65)In_(0.05)N layer 18, and source and drain electrodes 8and 10 each have ohmic contact with Al_(0.3)Ga_(0.65)In_(0.05)N layer18.

Note that source and drain electrodes 8 and 10 can be formed of a filmof metal that is formed of a Ti layer and an Al layer deposited in thisorder, a film of metal that is formed of a Hf layer, an Al layer, a Hflayer, and an Au layer deposited in this order, or the like.

Subsequently Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 is partially etchedaway in plasma to partially expose a surface of DyN layer 17 to form arecessed gate, as shown in FIG. 8 showing a schematic cross section.Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 can be etched in plasma for examplein chlorine gas by an inductively coupled plasma (ICP) etcher.

On at least a portion of the exposed surface of DyN layer 17 gateelectrode 9 formed for example of WN is disposed to have schottkycontact with DyN layer 17. A wafer having provided with gate electrode 9is divided into chips to provide the hetero junction field effecttransistor shown in FIG. 5.

In the present embodiment Al_(0.25)Ga_(0.7)In_(0.05)N layer 16, DyNlayer 17 and Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 provide a 3-layerbarrier layer. The center, DyN layer 17 is thus etched in plasma at arate significantly smaller than Al_(0.3)Ga_(0.65)In_(0.05)N layer 18. Ifthe layer is etched in plasma at a slightly varying rate or for aslightly increased period of time, the underlying,Al_(0.25)Ga_(0.7)In_(0.05)N layer 16 is not etched in plasma. This canleave Al_(0.25)Ga_(0.7)In_(0.05)N layer 16 well controlled to have athickness required for normally off operation, and thus form a recessedgate with good repeatability.

The hetero junction field effect transistor thus fabricated can be anormally off hetero junction field effect transistor excellent inin-plane uniformity (i.e., the uniformity of the voltage (0V fornormally off) being applied to the gate electrode of each heterojunction field effect transistor that is obtained from a single waferwhen the transistor is pinched off) and run-to-run uniformity (i.e., theuniformity of the voltage (0V for normally off) being applied to thegate electrode of each of hetero junction field effect transistors thatare obtained from different wafers identical in structure when thetransistor is pinched off).

Note that Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 can be etched in plasmafor a period of time that can be determined as follows: A period of timeallowing Al_(0.3)Ga_(0.65)In_(0.05)N layer 18 to be etched in plasmaentirely in depth is previously calculated and for example thecalculated period of time is then increased by a few tens percents, andfor such a longer period of time than the previously calculated periodof time the layer may be etched in plasma. For example, if a period of60 seconds is required to etch Al_(0.3)Ga_(0.65)In_(0.05)N layer 18having a thickness of 25 nm in plasma, the layer is etched in plasma for80 seconds. As DyN layer 17 acts as an etching stopper layer, theunderlying, Al_(0.25)Ga_(0.7)In_(0.05)N layer 16 can remain to have apredetermined thickness uniformly.

Furthermore while in the above description the second layer isimplemented by DyN layer 17, the second layer is preferably implementedby a layer of a nitride of at least one rare earth element selected fromthe group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm,Yb and Lu. A nitride of these rare earth elements providessemiconductor. Thus the second layer thus configured also does notimpair the transistor in characteristic and also has a tendency to serveas an effective etching stopper layer.

Furthermore in the present invention preferably the second layer formedof the nitride of the rare earth element(s) has a thickness of at most 5nm. If the second layer has a thickness exceeding 5 nm it wouldsufficiently serves as an etching stopper layer. However, this providesa tendency that a layer underlying a gate electrode has a thicknesslarger than required to provide a normally off transistor, and suchtransistor may not be provided.

Furthermore in the present invention preferably the third layer isformed of an n-type, nitride based, group III-V compound semiconductor.If the third layer is formed of the n-type, nitride based, group III-Vcompound semiconductor, there is a tendency that the ohmic contact withthe source and drain electrodes disposed on the third layer can bereduced in resistance. This can contribute to reduced on-stateresistance. Furthermore if the second layer has a band gap smaller thanthe third layer the third layer is preferably formed of n type, nitridebased, group III-V compound semiconductor to supply sufficient electronsto supplement those trapped in the second layer.

Furthermore in the present invention the plasma etching is preferablydone with a gas containing chlorine or fluorine, since a chloride of arare earth element and a fluoride of a rare earth element is low invapor pressure, and the plasma etching rapidly drops in rate, andaccordingly the second layer formed of a nitride based, group III-Vcompound semiconductor containing a rare earth element or a nitride of arare earth element has a tendency to serve as an effective etchingstopper layer.

The present invention can thus provide a hetero junction field effecttransistor allowing a recessed gate to be formed with good repeatabilityand a method of fabricating such hetero junction field effecttransistor.

Although the present invention has been described and illustrated indetail, it is clearly understood that the same is by way of illustrationand example only and is not to be taken by way of limitation, the spiritand scope of the present invention being limited only by the terms ofthe appended claims.

1. A hetero junction field effect transistor comprising: a first layerof a nitride based, group III-V compound semiconductor; a second layerof a nitride of a rare earth element, overlying said first layer; a pairof third layers of a nitride based, group III-V compound semiconductor,overlying said second layer, said third layers being spaced from eachother; a gate electrode disposed between said third layers at least aregion of said second layer; and a source electrode overlying one ofsaid third layers and a drain electrode overlying an other of said thirdlayers.
 2. The hetero junction field effect transistor according toclaim 1, wherein said rare earth element is at least one selected fromthe group consisting of Ce, Pr, Nd, Pm, Sm, Eu, Gd, Th, Dy, Ho, Er, Tm,Yb and Lu.
 3. The hetero junction field effect transistor according toclaim 1, wherein said third layer is formed of an n type, nitride based,group III-V compound semiconductor.
 4. The hetero junction field effecttransistor according to claim 1, wherein said second layer has athickness of at most 5 nm.